Someday, when the passions of this election have subsided, historians and analysts of American foreign policy may fasten on a remarkable passage in John Kerry's nomination speech. "As president," Kerry declared, "I will bring back this nation's time-honored tradition: The United States of America never goes to war because we want to; we only go to war because we have to. That is the standard of our nation." The statement received thunderous applause at the convention and, no doubt, the nodding approval of many Americans of all political leanings who watched on television.

Only American diplomatic historians may have contemplated suicide as they reflected on their failure to have the smallest influence on Americans' understanding of their own nation's history. And perhaps foreign audiences tuning in may have paused in their exultation over a possible Kerry victory in November to reflect with wonder on the incurable self-righteousness and nationalist innocence the Democratic candidate displayed. Who but an American politician, they might ask, could look back across the past 200 years and insist that the United States had never gone to war except when it "had to"?

The United States has sent forces into combat dozens of times over the past century and a half, and only twice, in World War II and in Afghanistan, has it arguably done so because it "had to." It certainly did not "have to" go to war against Spain in 1898 (or Mexico in 1846.) It did not "have to" send the Marines to Cuba, Haiti, the Dominican Republic, Mexico and Nicaragua in the first three decades of the 20th century, nor fight a lengthy war against insurgents in the Philippines. The necessity of Woodrow Wilson's intervention in World War I remains a hot topic for debate among historians.

And what about the war Kerry himself fought in? Kerry cannot believe the Vietnam War was part of his alleged "time-honored tradition," or he would not have thrown his ribbons away. But America's other Cold War interventions in Asia, Latin America and the Middle East are also problematic. Most opponents of the Vietnam War, like Kerry, believed it was symptomatic of a larger failure of U.S. foreign policy stemming from what Jimmy Carter memorably called Americans' "inordinate fear of communism." The other Cold War interventions were premised on the same "misguided" anti-communism and the concomitant democratic idealism, that pulled Kerry's hero, John F. Kennedy, into Vietnam. The United States, by this reckoning, did not "have to" go to war in Korea in 1950. Nor could a post-Vietnam Kerry have considered Lyndon Johnson's 1965 intervention in the Dominican Republic necessary. Or has Kerry now retroactively accepted the Cold War justification for these interventions that he once rejected?

Then there were the wars of the post-Cold War 1990s. The United States did not "have to" go to war to drive Saddam Hussein from Kuwait. No one knows that better than Kerry, who voted against the Persian Gulf War, despite its unanimous approval by the U.N. Security Council. Nor could anyone plausibly deny that the Clinton administration's interventions in Haiti, Bosnia and Kosovo were wars of choice. President Bill Clinton made the right choice in all three cases, but it was a choice.

Why is Kerry invoking an American "tradition" that does not exist?

Perhaps he's distorting American history simply to cast the Bush administration and the war in Iraq in the harshest possible light. But maybe Kerry is not being cynical. Perhaps, finally, he is saying what he really believes and not what American policy has been, but what it should be.

The doctrine Kerry enunciated on Thursday night, after all, was the doctrine initially favored by the antiwar movement and the mainstream of the Democratic Party after the debacle of Vietnam. "Come home, America" was the cry of those who believed America had corrupted both the world and itself in "wars of choice" in Vietnam and elsewhere.

Advocates of this doctrine did not propose a "return" to some mythical American past. Rather, they proposed a radical departure onto a very different course in American foreign policy. Their goal was a retraction of American power and influence from around the globe. Nor did they have any doubt that their view of America was patriotic. They would cleanse America of its sins.

Would it really be surprising if John Kerry, whose life and thought were so powerfully shaped by his Vietnam experience, now returned to the view of American foreign policy which that experience led him to three decades ago? There seems to be a conspiracy on both sides in this campaign not to take Kerry seriously as a man of ideas and conviction. But the fact that he has waffled so visibly on Iraq may be the best proof of his commitment to the beliefs about American foreign policy he came to hold in the 1970s.

Maybe Kerry's real act of cynicism was his vote for the Iraq war in the fall of 2002. With that vote, he ignored everything he believed he had learned from his Vietnam experience. In retrospect, he may feel that he sold his soul to make himself electable. In the months since the war, Kerry has had to pretend he did the right thing, not only because a politician dare not admit error but because his political advisers believe that in a post-Sept. 11 world most of the electorate does not want an "antiwar" president. Throughout the long months of the campaign, Kerry disciplined himself to sound like a hawk. But in his heart, based on all he learned during the formative years of his life, Kerry is not a hawk. At the Democratic National Convention, John Edwards followed the script. Kerry followed his heart.

The ironies abound. Three decades ago, Kerry came out in opposition to the war he had fought in Vietnam. Today, Kerry extols that service so that he may safely, patriotically distance himself from the war in Iraq that he had supported.

If Kerry has revealed himself in an unusual moment of honesty, it's time everyone took an equally honest look at where he would lead the country if elected. Kerry's "doctrine of necessity," if seriously intended, would entail a pacifism and an isolationism more thorough than any attempted by a U.S. government since the 1930s. It would rule out all wars fought for humanitarian ends, all interventions to prevent genocide, to defend democracy or even, as in the case of the Persian Gulf War, to uphold international law against aggression. For those are all wars of choice.

For someone who professes to seek better relations with the rest of the world, Kerry's doctrine of necessity would base American foreign policy on narrow, selfish interests far more than the alleged "unilateralism" of the Bush administration. Some Europeans have been quietly worrying that what they consider Bush's overambitious foreign policy will be followed in the United States by an isolationist backlash. After hearing Kerry's speech, they may worry a bit more.

Robert Kagan, a senior associate at the Carnegie Endowment for International Peace, writes a monthly column for The Post.